The present invention relates to an electrophotographic image forming apparatus and, more particularly, to an image forming apparatus capable of adjusting the amount of a low friction agent to be applied to the surface of a photoconductive element on the basis of image quality.
It is a common practice with an electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus to scan a uniformly charged photoconductive element with a beam modulated by image data so as to electrostatically form a latent image thereon. The latent image is developed by toner fed from a developing section to turn out a corresponding toner image. The toner image is transferred from the photoconductive element to a paper or similar recording medium at an image transfer station and then fixed by heat and pressure at a fixing station. After the image transfer, the toner left on the photoconductive element is removed by, e.g., a cleaning blade located at a cleaning station.
It has been customary with the above apparatus to apply a lubricant to the surface of the photoconductive element. The lubricant lowers the coefficient of friction on the surface of the photoconductive element and thereby prevents needless toner from depositing on the element, so that the background of the resulting image is free from contamination. The lubricant is applied and then scraped off in order to refresh the photoconductive element at all times, thereby extending the life of the element.
Particularly, in an image forming apparatus of the type removing toner left on a photoconductive element with a cleaning blade, the low friction agent applied to the element successfully lowers the coefficient of friction between the element and the blade at the time of cleaning and thereby extends the life of the element.
Specifically, the wear of a photoconductive layer formed on the photoconductive element is one of major factors determining the life of the element. Should the photoconductive layer become worn by more than a preselected amount, the electric characteristic of the photoconductive element would vary too much to effect an adequate image forming process. While the wear occurs at all the positions where the photoconductive element contacts a developing section, an image transfer section and so forth, the cleaning blade dynamically removing the toner from the element is most critical. The wear at the other stations does not have substantial influence on the life of the photoconductive element. As for the cleaning station, the wear is generally classified into two different kinds of wear, i.e., one ascribable to a shearing force acting between the blade and the photoconductive element and the other ascribable to the toner behaving like a whetstone between the blade and the element. Factors determining such wear include the structural strength of the photoconductive element, the contact pressure of the blade contacting the element, the composition of toner particles, and the coefficient of friction of the surface of the element.
Therefore, by reducing the coefficient of friction of the photoconductive element with the low friction agent, it is possible to reduce the wear at the cleaning station for thereby extending the life of the element. In light of this, an image forming apparatus capable of applying a lubricant evenly to the surface of a photoconductive element is taught in, e.g., Japanese Patent Laid-Open Publication No. 8-20226. The apparatus is directed toward the obviation of background contamination ascribable to the unevenness of the lubricant
However, the apparatus taught in the above document has some room for further improvement in the image quality aspect, as follows. The low friction agent applied to the photoconductive element lowers the coefficient of friction and thereby reduces adhesion between the toner and the element at the time of development. This prevents the toner from being transferred to the photoconductive element in an expected manner. This is particularly true with, e.g., a two-ingredient type developer developing a latent image in contact with the photoconductive element. Specifically, when particles particular to such a developer contact the surface layer of the photoconductive element, they exert a dynamic force and are likely to scrape off the toner successfully transferred to the element or to displace the toner image away from the expected position. This phenomenon occurs little so long as the coefficient of friction on the surface of the photoconductive element is high. However, when the coefficient of friction is reduced by the low friction agent, the above phenomenon becomes noticeable, depending on the amount of the low friction agent, i.e., the decrease of the coefficient of friction. This phenomenon does not occur in a two-level fashion, i.e., does not occur suddenly on the decrease of the coefficient of friction to a certain value. That is, images sequentially vary in accordance with the variation of the coefficient of friction of the surface of the photoconductive element and appear defective to the eye when varied over a certain degree.
Moreover, various matters deposit on the photoconductive element during development and include NOx, SOx and other ion oxides ascribable to ozone produced at a charging station and an image transfer station due to discharge and dielectric breakdown. Such matters are so hydrophilic, they gather water molecules existing in the air. As a result, the electric resistance of the surface layer of the photoconductive element is lowered. This prevents a latent image formed on the photoconductive element by a beam from holding its charge and thereby disturbs the image.
Usually, the above matters are scraped off by the cleaning blade used to clean the photoconductive element and are not critical in practice. However, the low friction agent applied to the photoconductive element for reducing friction reduces the coefficient of friction and therefore a shearing force to act between the photoconductive layer of the photoconductive element and the blade. This obstructs the removal of the undesirable matters from the photoconductive element and thereby brings about defective images. Again, such defective images do not occur in a two-level fashion.
To solve the above problems, it is necessary to control the amount of the low friction agent to be applied to the photoconductive element for varying the coefficient of friction of the surface of the element.